Air rectification with multiple reboilers and condensers

ABSTRACT

This invention relates to a method of fractional distillation in which a feed, preferably air, in introduced into a distillation column having distributed up its height a number of reboilers and condensers which are arranged so as to work the column approximately reversibly and obtain a liquid bottom fraction and a top gaseous fraction. At least a part of the gaseous top fraction and at least a part of the feed are used as the heating medium in reboilers, and at lease part of the liquid bottom fraction is used a s the cooling medium in at lease one condenser positioned above the highest reboiler.

sept- 1971 R. STOKLOSINSKI 3,605,423

AIR RECTIFICAIION WITH MULTIPLE REBOILERS AND counnusans Filed llay 28, 1968 SQ w United States Patent U.S. Cl. 62--28 8 Claims ABSTRACT OF THE DISCLOSURE This invenion relates to a method of fractional distillation in which a feed, preferably air, is introduced into a distillation column having distributed up its height a number of reboilers and condensers which are arranged so as to work the column approximately reversibly and obtain a liquid bottom fraction and a top gaseous fraction. At least a part of the gaseous top fraction and at least a part of the feed are used as the heating medium in reboilers, and at least part of the liquidbottom fraction is used as the cooling medium in at least one condenser positioned above the highest reboiler.

This invention relates to improvements in or relating to gas separation, and is especially concerned with the separation of air by fractional distillation to obtain an oxygen product.

According to the invention there is provided a method of fractional distillation in which a feed, normally air, is introduced into a distillation column having distributed at intervals up its height a number of reboilers and condensers which are arranged so as to work the column ap proximately reversibly and obtain a liquid bottom fraction, normally oxygen, and a gaseous top fraction, normally nitrogen, at least a part of the gaseous top fraction and at least a part of the feed being used as the heating medium in the or each reboiler, and at least part of the liquid bottom fraction being used as the cooling medium in at least one condenser, positioned above the highest reboiler.

The method according to the invention is extremely simple and because the column is worked approximately reversibly it is eflicient.

According to a preferred embodiment of the invention, a part of the gaseous top fraction is compressed, cooled and then used as the heating medium in a reboiler where it condenses and it is then throttled before it is returned to the top of the column, and a part of the air feed is compressed, cooled and then used as the heating medium in another reboiler where it condenses, and it is then throttled and fed to the column. Also the liquid oxygen product is withdrawn from the column, is throttled or is divided into two or more streams throttled to different pressures, and the or each stream is passed through one of the column condensers before it constitutes the oxygen product. By working in this way heat can be added and withdrawn at various points up the height of the column and so the column can be worked approximately reversibly.

The invention also includes a plant for operating in accordance with the above method.

The invention will now be described, by way of example, with reference to the accompanying drawing, which is a flow sheet of a process according to the invention.

Atmospheric air is drawn into the plant through a filter 10 and is compressed in a compressor 12 to the low excess pressure required to overcome friction losses within the 3,695,423 Patented Sept. 20, 1971 plant. The compression step causes heating of the air and it is therefore cooled in an after cooler 16 by heat exchange with at least a part of the oxygen product (not shown), and then with the nitrogen product stream. Finally it is cooled to about 40 F. in a refrigerator 18 so as to condense water vapour and thereby assist in the drying of the air. The refrigeration necessary to operate the refrigerator 18 can be obtained from an outside source. The cooled air is next purified by passing it through one of a pair of alternately operating driers 20. Each drier has an upper bed of desiccant, for example silica gel, and a lower bed of molecular sieves. Most of the water vapour is absorbed in the upper bed of silica gel while the molecular sieves serve to absorb the remainder of the moisture and also the carbon dioxide in the air. The purification of the air in this way ensures eflicient and uninterrupted running of the plant.

The clean dry carbon dioxide free air from the driers is passed through a heat exchanger 22 in which it is cooled to a temperature near its dew point by heat exchange with the products from the distillation column 24. The cooled air is then fed to an intermediate position in the column 24, and it is separated into a top nitrogen product and a bottom oxygen product. The column 24 is run approximately reversibly by having distributed throughout its length a number of reboilers 26 and 27 and condensers 28 and 29 to add and remove heat and so provide reboiling and reflux at various points up the height of the column.

In the embodiment shown, the distribution of reflux is obtained using streams of nitrogen and oxygen products and air in conjunction with three compressors 30 to 32. A still better approach to reversible operation could be obtained by employing more reboilers and condensers and more compressors. In practice, however, it is not usually necessary or economical to use more than are shown.

The gaseous nitrogen top product is taken from the column to the heat exchanger 22 to cool the incoming air and then some of the nitrogen is fed to the compressor 30 where it is compressed to a medium pressure and returned to the heat exchanger for cooling, while the remainder of the nitrogen is led first to the cooler 16 m is then passed through the drier 20 which is not being used to reactivate it. The compressed nitrogen returned to the heat exchanger is cooled to near its dew point at that pressure and it is then taken to the reboiler 27 where it condenses in heat exchange with boiling reflux inside the column. The condensate is then throttled through a valve 34 to the pressure inside the column and is itself introduced at the top of the column as reflux.

The purified air from the driers 20 is split into two streams. As previously described, the larger stream is fed directly to the column through the heat exchanger 22. The other stream is compressed in the compressor 32 before being passed through the heat exchanger 22 and introduced into the reboiler 26. There it condenses in heat contact with the boiling reflux passing down the column, and the condensed liquid is throttle through a valve 36 to the pressure in the column and introduced near the top of the column as reflux.

Liquor oxygen product in the sump of the column is removed by a line 38 and divided into two streams which are throttled in valves 40 and 42 to a lower pressure, the valve 40 being arranged to throttle its stream to a lower pressure than the valve 2. The streams from the valves 40 and 42 are fed respectively to the condensers 29 and 28 and both liquid streams vaporise in these boilers by heat contact with the vapours in the column, part of which are condensed for reflux. The oxygen vapour product streams are taken from the condensers 28 and 29 to the heat exexchanger 22 and the after cooler 16 where they are warmed to ambient temperatures and passed to a compressor 31 which restores the streams to atmospheric pressure and also combines the streams. Part of the gaseous product from the compressor 31 is returned via the heat exchanger 22 to the sump of the column while the balance constitutes the oxygen product.

If desired the oxygen stream from the sump could have been divided into more than two streams which would pass through more than two condensers.

In order to prevent any build-up of impurities in the boiling oxygen in the two condensers 28 and 29 a small stream of liquid oxygen is withdrawn alternately from one or other or from both condensers by a lift pump 44 and circulated through one of a pair of alternately operating purifiers 46. The purifier not being used is reactivated by passing through it a stream of nitrogen product. The purified oxygen stream is returned to the condenser or condensers.

The plant just described will only produce oxygen of at the most about 50% purity. Pure oxygen can, however, be easily produced by adopting the simple expedient of extending the column at the sump and withdrawing sump vapour, compressing it and using it to reboil some of the sump liquid before it is throttled and returned to the top of the extended sump. This arrangement is described in detail in FIG. 2 of my Pat. No. 1,006,499, and a similar alternative arrangement is described in my Pat. No. 1,057, 037.

The plant can also be made to produce pure nitrogen by withdrawing a stream of impure nitrogen gas from near to the top of the column .as a third stream and venting this to the atmosphere as a waste purge.

A liquid oxygen product can also be produced or both liquid and gaseous products can be produced by withdrawing from the plant oxygen liquid from the sump. The necessary extra refrigeration for the process can then be introduced at the condensers where the sump liquid is normally vaporized by withdrawing part of the vapours from the column at that point, condensing them in an auxiliary refrigerator and then returning them to the column at the same level, or by other suitable means.

The plant just described is relatively simple and yet is eflicient. The column is operated substantially reversibly and its operation need require only three compressors compared with the six preferred for the column shown in my Pat. No. 1,006,499. Also the amount of nitrogen stream handled by the compressor 30 need not be more than 30% of the initial air feed.

I claim:

1. A method of fractionally distilling the air comprising the steps of:

(a) preparing a supply of dry substantially carbon dioxide free air,

(b) dividing said supply of air into first and second parts,

() cooling said first part of said dry substantially carbon dioxide free air by passing it through a heat exchanger,

(d) feeding said first part to a single distillation column, said column having distributed up its height at least two reboilers and a plurality of condensers spaced from one another with the highest reboiler below the lowest condenser,

(e) operating said column so as to separate said air fed thereto into a top gaseous nitrogen product and a bottom liquid oxygen product,

(if) compressing said second part of said air supply,

(g) cooling said compressed second part and passing it through one of said reboilers,

(h) throttling said second part after passage through said one reboiler and introducing said throttled second part into said column,

(1) dividing said top gaseous nitrogen product into a first and second parts,

(j) passing said first part of said nitrogen product through said heat exchanger to cool said air supply,

(k) compressing said second part of said nitrogen product, and passing said compressed product through annother of said reboilers, throttling it and returning it to a point adjacent the top of said column,

(1) withdrawing said liquid oxygen product, dividing it into a plurality of portions, throttling each of said plurality of portions to a different pressure, and supplying each throttled portion to one of said plurality of condensers,

(m) passing said oxygen products, from said plurality of condensers to said heat exchanger to cool said supply of dry substantially carbon dioxide free air, and

(n) compressing said oxygen product from said heat exchanger to give product oxygen gas.

2. A method according to claim 1 further comprising dividing said liquid oxygen into two streams, throttling each stream to different lower pressures, passing each stream to one condenser as the cooling medium, compressing said streams from said condensers to the same pressure and combining said stream to give the product oxygen gas.

3. A method according to claim 2 in which part of said combined oxygen gas product is cooled and recycled to the base of said column.

4. A method according to claim 1 further comprising the steps of withdrawing oxygen gas from the lower part of said column, compressing said gas, passing said gas through a sump reboiler in which it becomes substantially completely condensed, and returning said condensed oxygen to the lower part of said column as liquid reflux.

5. A method according to claim 1 further comprising the steps of throttling a part of the liquid oxygen bottom product and evaporating it in a condenser positioned in the lower part of the column to provide a liquid reflux in that part of the column.

6. A plant for the fractional distillation of air to separate the air into oxygen and nitrogen products comprising:

(a) a single distillation column for separating air into a top gaseous nitrogen fraction and a bottom liquid oxygen fraction,

(b) at least two reboilers spaced from one another up the height of said column,

(c) a plurality of condensers spaced from one another up the height of said column and positioned so as to be spaced above the highest reboiler,

(d) a heat exchanger,

(e) means for supplying a first stream of dry substantially dioxide free air to said heat exchanger to be cooled therein,

(f) means for supplying first part of said cooled air from said heat exchanger to said column for distillation,

(g) means for compressing, a second stream of dry substantially carbon dioxide free air, cooling said second stream, passing it through one of said reboilers positioned in said column and thereafter passing it to said column for distillation,

(h) means for withdrawing a first part of said nitrogen top product and passing it through said heat exchanger to cool said first and second supplier of air,

(i) means for removing a second part of said gaseous nitrogen top fraction from said column, compressing it, passing it through one of said reboilers positioned in said column, throttling said second part and returning it to the top of said column, and

(j) means for removing said bottom liquid oxygen fraction and dividing it into a plurality of oxygen portions, means for throttling each of said plurality of portions to a different pressure and passing each throttled oxygen portion through. one of said plurality of condensers positioned in said column, means for passing each of said plurality of a portion from said condensers through said heat exchanger to cool said first and second supplies of air, and means for compressing said plurality of a portion from said heat exchanger to provide oxygen product gas.

7. A plant according to claim 6 having two reboilers, means for dividing said bottom liquid oxygen fraction into first and second parts, first throttling means for reducing the pressure of said first part, second throttling means for reducing the pressure of said second part to a lower pressure than said first part, means for passing said throttled first part to the lower of said two reboilers, means for passing said throttled second part to the upper of said two boilers, means for passing both parts from said reboilers through said heat exchanger, and means for compressing both parts to the same pressure and combining them to provide product oxygen gas.

8. A plant according to claim 7 further comprising means for returning part of said product oxygen gas to said column.

References Cited UNITED STATES PATENTS NORMAN YUDKOFF, Pn'mary Examiner A. F. PURCELL, Assistant Examiner US. Cl. X.R. 62-18, 26 

